This invention relates to compositions useful for preserving timber products. The timber products to which the present invention is directed are typically foundation piles, utility poles, posts, fences, decks, railroad ties, marina structures and other types of products subject to decay due to insect pests, fungi and weathering. The compositions of the invention provide improved wood treating solutions having enhanced stability when compared to conventional CCA type solutions. In addition, these compositions, when used to preserve wood products, retard the hardness usually associated with CCA type solutions, resulting in less cracking and improved climbability.
The two main types of preservatives used to treat timber products include oil-based preservatives such as coal tar creosote, solutions of creosote with coal tar or petroleum oils, and solutions of preservative chemicals, such as pentachlorophenol dissolved in a suitable organic Carrier; and water-based preservatives such as acid copper chromate solution (ACC), chromated copper arsenate solution (CCA) and ammoniacal copper arsenate solution (ACA). CCA solutions are preferred since they chemically react in the wood to form compounds which are toxic to both fungi and insects.
However, CCA solutions have been known to impart undesirable characteristics to treated wood such as hardening due to chemical fixation of Cr (VI) in the wood, and various attempts have been made to improve CCA type wood treating solutions to increase softness of the treated wood. In addition, the use of reducing agents or other additives in CCA solutions tends to raise the pH of the solution above about 2, causing premature precipitation of chromated copper arsenate from wood treating solutions prior to or during use, resulting in unacceptable treated products and significant costs associated with waste disposal or replacement of the solutions.
U.S. Pat. No. 4,567,115 describes a method of treating wood, comprising the step of exposing wood to an aqueous solution consisting essentially of water, hexavalent chromium, copper, arsenic and polyethylene glycol. In a particular embodiment, a 10% concentration of 1000 molecular weight polymer of ethylene glycol (PEG 1000) is added to the standard diluted CCA solution in a manner so that a stable solution is obtained. The useful range of PEG solutions is described as being in the 500 to 2000 molecular weight range.
U.S. Pat. No. 4,847,002, issued to the same inventor of the above patent, describes another attempt at improving the stability of CCA-type solutions. The patent describes a wood preservative solution comprising a CCA-type formulation diluted with water and an added amount of fluoride ion sufficient to stabilize the solution against precipitation without retarding conversion of hexavalent chromium to trivalent chromium in the wood matrix. The patent also claims a wood preserving solution further consisting essentially of polyethylene glycol. The molecular weight range of the polyethylene glycol is between 100 and 2000, in particular 500 to 2,000 and more particularly 1,000.
Advances have been made in improving CCA solutions. However, problems associated with stability of these solutions still exist. In addition, resistance to the use of utility poles treated with CCA persists due to problems associated with hardness of the treated wood and resulting problems associated with cracking of CCA treated wood.
The need exists for stable wood preservative solutions which combine the preservative properties of CCA type solutions with the ease of climbing associated with oil-based solutions.